1. Field of the Invention
The present invention relates to a method for creating symmetric-identical preamble of Orthogonal Frequency Division Multiplexed signals and a method for synchronizing symbol and frequency of OFDM signals by using the symmetric-identical preamble. More particularly, the present invention relates to a method for creating symmetric-identical preamble of OFDM signals and for synchronizing symbol and frequency of OFDM signals by using the created symmetric-identical preamble, and a method for synchronizing symbol and frequency of OFDM signals by using the symmetric-identical preamble.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (hereinafter, it is referred as OFDM) is a very effective way to transmit data through a channel. In order to transmit the data, OFDM uses a plurality of sub-carrier frequencies within a channel bandwidth. Such sub-carrier frequencies are allocated bandwidth to maximize the efficiency of bandwidth better than other conventional methods, such as, Frequency Division Multiplexing (FDM), so that they can avoid inter-carrier interference (ICI) very well. Moreover, coating the sub-carrier frequencies with the data reduces any further loss due to frequency-selective fading, and brings more advantages of frequency diversity.
In the OFDM system the frequency offset and synchronizing symbol should be considered to properly demodulate transmitting signals. If the starting point of the symbol is not properly detected, the inter-symbol interference (ISI) occurs, and the transmitting signals cannot be restored well. In general, the starting point of the symbol is found using a correlation value coefficient between the receiving signals. At this time, a specific signal of a specific preamble is applied in order to obtain the correlation value.
The conventional method for synchronizing symbol and frequency of OFDM signals is illustrated with reference to FIG. 1 through FIG. 4.
FIG. 1 is a block diagram of a conventional OFDM signal receiving set suggested by Schimidle.
Referring to FIG. 1, the OFDM receiver receives data, r(n), through a RF (radio frequency) receiver (not shown). Then, the received OFDM signals are inputted to the self-correlation unit (20) to restore baseband data bit. The self-correlation unit (20) delays the received data, r(n), as long as a half of the OFDM symbol length (N), or N/2. The self-correlation unit (20) also calculates a delayed signal, r*(k-D) and a self-correlation value of the received signals, r(n).
Peak detection unit (40) calculates the mean of N/2 self-correlation values, and detects a maximum of the mean values that are obtained per sample. Time/Frequency synchronization unit (50) obtains a time domain synchronization based on the maximum mean value that is detected. After the precise symbol synchronization in the time domain is obtained, the frequency offset problem should be solved next.
The frequency offset often occurs when the generator of a receiver is inaccurate. The frequency offset perverts the amplitude and phase of the demodulate signals, and causes inter-sub channel interference that consequently deteriorates the entire capacity of the OFDM system. Normally, the frequency offset is divided into a coarse frequency offset and a fine frequency offset.
The time/frequency synchronization unit (50) estimates and compensates the fine frequency offset of the sub-carrier using a guard interval in the time domain. In order to compensate the coarse frequency offset, the signal that the fine frequency offset of the sub-carrier is compensated, and the received signal r(n), go through the inverse Fourier transformation at IFFT (Inverse Fast Fourier Transform) unit (60).
The correlation values between the inverse Fourier transformed signals and N differential signals v(k) are calculated, and the mean of the correlation values are obtained in order to detect the maximum correlation values in each sample. The coarse frequency synchronization unit (80) obtains an integral frequency synchronization from the detected maximum values.
FIG. 2 is a diagram illustrating a structure of the conventional preamble (PR1) of OFDM signals suggested by Schimidle.
As shown in FIG. 2, the preamble (PR1) for calculating OFDM signals offset using the conventional method suggested by Schimidle includes N number of samples, and, the identical sample layer is arrayed at intervals of N/2. That is, the preamble (PR1) for calculating the offset has the structure of [APR1, APR1], and the distance between the same sample layers is N/2 for all. The arrow A in FIG. 2 indicates the correlation between the samples with the correlation coefficients to be calculated.
The self-correlation unit (20) calculates a N/2 delayed signal and a self-correlation value in every sample, and then outputs them to the peak detection unit (40).
Suppose that there are 64 sub-carriers, and the experiment was conducted in the surroundings of additive white Gaussian noise (AWGN) at 5 dB. Here, although the self-correlation value is expected to have the maximum value at the 64th sample, the truth is that a greater correlation coefficient is obtained around the 64th sample. This means that the error rate in symbol timing synchronism around the sample where the maximum value is supposed to exist is considerably high due to other noises and influences of the channel.
Supposing that the precise starting point is found when m is zero in Formula 1 for correlation coefficients, the correlation metric P(d), can be expressed as follows in the case that the d sample is offset.
                                                                        P                ⁡                                  (                  d                  )                                            =                            ⁢                                                                                                            ∑                                              n                        =                        0                                                                                              N                          2                                                -                        1                                                              ⁢                                                                                  ⁢                                                                  r                        ⁡                                                  (                                                      n                            +                            d                                                    )                                                                    ⁢                                                                        r                          *                                                ⁡                                                  (                                                      n                            +                            d                            +                                                          N                              2                                                                                )                                                                                                                                      2                                                                                        =                            ⁢                                                                                                          ∑                                              n                        =                        0                                                                                              N                          2                                                -                        1                        -                        d                                                              ⁢                                                                                  ⁢                                                                  r                        ⁡                                                  (                                                      n                            +                            d                                                    )                                                                    ⁢                                                                        r                          *                                                ⁡                                                  (                                                      n                            +                            d                            +                                                          N                              2                                                                                )                                                                                                      +                                                                                                                                        ⁢                                                      ∑                                          n                      =                                                                        N                          2                                                -                        d                                                                                                            N                        2                                            -                      1                                                        ⁢                                                                          ⁢                                                            r                      ⁡                                              (                                                  n                          +                          d                                                )                                                              ⁢                                                                                            r                          *                                                ^                                            ⁡                                              (                                                  n                          +                          d                          +                                                      N                            2                                                                          )                                                                                                                        2                                                          [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          1                ]            
where d is the number of samples that are offset.
Because the receiving signals r(n+d) and
            r      *        ^    ⁡      (          n      +              N        2            +      d        )  belong to different symbols respectively, there should be no correlation between them. However, in the Formula 1, r(n+d) and
            r      *        ^    ⁡      (          n      +              N        2            +      d        )  correlates each other. Therefore, if the number of samples that are offset is small, it is very difficult to distinguish the precise symbol starting point from the correlation value.
On the other hand, the conventional method for synchronizing symbol and frequency of OFDM signals using preamble (PR1) suggested by Schimidle uses the absolute values for calculating the correlation metric P(d). Accordingly, when AWGN is included in the samples that are offset, the maximum correlation value is found around the actual starting point of the symbol.
As for estimating the frequency offset, however, since the conventional method for synchronizing symbol and frequency of OFDM signals using preamble (PR1) suggested by Schimidle has the constant sample interval, i.e., N/2, the fine frequency offset and the coarse frequency offset should be estimated separately.
FIG. 3 is a diagram illustrating a conventional preamble (PR2) structure of OFDM signals suggested by Minn.
Referring to FIG. 3, in the conventional preamble (PR2) structure of OFDM signals suggested by Minn, the preamble (PR2) includes four sub-preambles such as [APR2, APR2, −APR2, −APR2]. Namely, in the preamble (PR2) of Minn, the same sample stream is repeated at intervals of N/4. In addition, in the preamble (PR2) of Minn, the phases are inverted at internal of N/2. Therefore, the distance between the same samples is equally N/4.
As shown in the arrows B1 and B2 in the FIG. 3, the self-correlation unit (20) calculates the self-correlation value for the same samples between the sub-preamble having the same phase (APR2, APR2) and (−APR2, −APR2), in order to estimate the symbol timing.
FIG. 4 is a diagram illustrating a conventional preamble (PR3) structure of OFDM signals suggested by Morelli.
In the conventional preamble (PR3) structure of OFDM signals suggested by Morelli, the preamble (PR3) includes four sub-preambles such as [APR3, APR3, APR3, APR3]. Every sub-preamble includes the same samples with the length of N/4. Therefore, the distance between the same samples is equally N/4. In order to estimate the frequency offset, indicated in the arrow C of FIG. 4, a N/4 delayed sample and a self-correlation value for each sample are calculated.
As described above, the method for synchronizing symbol and frequency of the OFDM signals using the conventional preamble (PR1) suggested by Schimidle is disadvantageous in some ways. Firstly, the conventional preamble that is used to demodulate the OFDM signals cannot be used in the synchronizing modulation-demodulation method.
Secondly, whenever the signals are offset from the precise symbol starting point, the number of the signals without any correlation becomes equal to the number of offset samples. In this manner, the high error rate around the original symbol starting point results due to the influences of other noises and channels.
Thirdly, since the distance between the same samples is constant N/2 in the process of frequency offset estimation, the coarse frequency offset and the fine frequency offset should be estimated, respectively.
On the other hand, it is available to estimate the symbol timing using the preamble (PR2) of OFDM signals suggested by Minn, but it is impossible to estimate the frequency offset instead. Thus, there is a problem that a separate preamble should be added in order to estimate the frequency offset.
In contrast, it is available to estimate the frequency offset using the preamble (PR3) of OFDM signals suggested by Morelli, but it is impossible to estimate the symbol timing. Thus, there is a problem that a separate preamble should be added in order to estimate the symbol timing.